1. Field of the Invention
The present invention relates to a sliding component applied to a mechanical seal, a bearing, or another sliding part, for example. The present invention particularly relates to a sliding component of a sealing ring, a bearing, or the like, in which a fluid is present on sliding surfaces to reduce friction, and the fluid must be prevented from leaking out from the sliding surfaces.
2. Background Art
In a mechanical seal, which is one example of a sliding component, the contrary conditions “sealing” and “lubrication” must both be achieved in order to maintain airtightness over a long period of time. Particularly, in recent years there has been a rising demand for less friction in order to prevent leakage of sealed fluid and reduce mechanical loss, for the sake of environmental measures and the like. The means of reducing friction can be achieved by creating a so-called fluid lubrication state, in which dynamic pressure is generated by rotation between sliding surfaces and sliding occurs in the presence of a liquid membrane. However, in this case, positive pressure is generated between the sliding surfaces, and the fluid flows out of the sliding surfaces from the positive pressure portion. This is known as lateral leakage in a bearing, and is equivalent to the leakage in the case of a seal. The sealed fluid is located on the external peripheral side of the seal surface, the atmosphere is located on the internal peripheral side, and the internal-peripheral-side leakage rate when the fluid on the external peripheral side is sealed (known as the “inside type”) is expressed by the following formula.
                                          Q            =                          -                              ∫                                                      (                                                                                            h                          3                                                                          12                          ⁢                          η                                                                    ⁢                                                                        ∂                          p                                                                          ∂                          r                                                                                                                                              r                    =                    η                                                                                )                ⁢                              r            1                    ·                      ⅆ            θ                                              [                  Formula          ⁢                                          ⁢          1                ]            
Q: internal-peripheral-side leakage rate in sliding surface inside diameter r1
h: gap height
η: fluid viscosity
p: pressure
It is clear from the above formula that as fluid lubrication is promoted, dynamic pressure is generated, and a liquid membrane is formed; the pressure gradient ∂p/∂r on the internal peripheral end side increases, h increases, and as a result, the leakage rate Q increases.
Consequently, to reduce the leakage rate Q in the case of a seal, the gap h and the pressure gradient ∂p/∂r must be reduced.
From the above matters, in a conventional seal, a so-called compromise of sealing and lubrication, in which sealing performance is maintained, is achieved by reducing the liquid membrane thickness h to an extent that does not damage the sliding surfaces.
Consequently, in a seal used in an environment where contact causes immediate surface damage such as scorching, the result of prioritizing the dynamic pressure effect is that the liquid membrane thickness h increases and the leakage rate therefore increases. In a seal used in an environment in which direct contact does not readily cause problems even over a long period of time, the result of prioritizing sealing performance is that the gap h is small, the dynamic pressure effect is also small, and there is therefore a higher possibility of surface abrasion or damage due to direct contact, and a higher frictional coefficient. An example of the former structure is the invention disclosed in Patent Document 1, for example. This invention is a dry gas seal but can also be applied to a liquid seal, and although an excellent dynamic pressure effect is obtained, the leakage rate is extremely high. An example of the latter structure is a structure in which calcined carbon having excellent self-lubrication in a stationary ring side is used so that problems are unlikely to occur even with direct contact, and flat surfaces are sealed together. In another example, undulation or a spiral groove is implemented as a dynamic pressure generating mechanism (see Patent Documents 2 and 3, for example).
In a liquid seal, since viscosity is higher than gas, the dynamic pressure effect is obtained by the unevenness of minute asperities or roughness of the surfaces, even if the surfaces are flat. Therefore, structures that prioritize sealing performance are often used. To achieve both sealing and lubrication, a number of structures have been proposed which have a pumping effect of drawing leaked liquid back to the high-pressure side. Patent Document 4, for example, discloses a mechanism in which pumping is achieved by shear flow, due to a “barrier” of a different height being set up in advance between two rotating or static surfaces separated by a gap. In this mechanism, the structure is complicated because an initial gap must be provided mechanically, and since the gap is also present when no motion is occurring, a problem is encountered in that leakage occurs when no motion is occurring.
Non-patent Document 1 discloses a structure in which a high-pressure-side fluid is temporarily retained in a dam part, and after dynamic pressure is generated in a Rayleigh step bearing part, the fluid is returned to the high-pressure side. In this structure, since dynamic pressure is not generated until the liquid is retained in the dam part, sliding occurs along with direct contact immediately after rotation starts, and accordingly there is a risk of surface damage occurring during this time.
Furthermore, Non-patent Document 2 discloses a proposal of creating a pumping effect using a shear flow during rotation, due to a pumping groove being set up on the upstream side of a Rayleigh step. In this mechanism, a problem is encountered in that leakage occurs when no motion is occurring because a high-pressure side and a low-pressure side are joined by the pumping groove.
The present applicant has submitted for application, as an invention relating to a sliding component, an invention in which a sealed fluid is led into a sliding surface by suction means formed on a sealed fluid side of the sliding surface, and the led-in sealed fluid is stored via a dam part in two dimple parts formed in the sliding surface, one on a radially external peripheral side and one on a radially internal peripheral side, while the sealed fluid is simultaneously pumped in the dimple part on the radially internal peripheral side; whereby the sealed fluid is prevented from leaking out from a seal surface positioned nearer the radially internal peripheral side than the two dimple parts (see Patent Document 5). In this invention, among the two dimple parts, a pumping action is created in the dimple part on the radially internal peripheral side to prevent the sealed fluid from leaking out from the seal surface, but the dimple part on the radially internal peripheral side forms a closed space and therefore has no negative pressure. Therefore, it is not possible to prevent leakage of the fluid present on the sliding surface that is nearer the radially internal peripheral side than the dimple part. Specifically, it is possible to prevent leakage to a certain extent, but an increase in the leakage rate cannot be avoided.
As described above, there is no conventional technique for achieving both sealing and lubrication wherein there is no leakage when no motion is occurring, and during rotation including the start of rotation, fluid lubrication is in effect and leakage is prevented.